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.TH POPEN 3C "Dec 14, 2006"
.SH NAME
popen, pclose \- initiate a pipe to or from a process
.SH SYNOPSIS
.LP
.nf
#include <stdio.h>

\fBFILE *\fR\fBpopen\fR(\fBconst char *\fR\fIcommand\fR, \fBconst char *\fR\fImode\fR);
.fi

.LP
.nf
\fBint\fR \fBpclose\fR(\fBFILE *\fR\fIstream\fR);
.fi

.SH DESCRIPTION
.sp
.LP
The \fBpopen()\fR function creates a pipe between the calling program and the
command to be executed.  The arguments to \fBpopen()\fR are pointers to
null-terminated strings.  The \fIcommand\fR argument consists of a shell
command line.  The \fImode\fR argument is an I/O mode, either \fBr\fR for
reading or \fIw\fR for writing. The value returned is a stream pointer such
that one can write to the standard input of the command, if the I/O mode is
\fIw\fR, by writing to the file \fIstream\fR (see \fBIntro\fR(3)); and one can
read from the standard output of the command, if the I/O mode is \fBr\fR, by
reading from the file \fIstream\fR. Because open files are shared, a type
\fBr\fR command may be used as an input filter and a type \fIw\fR as an output
filter. A trailing \fBF\fR character can also be included in the \fImode\fR
argument as described in \fBfopen\fR(3C) to enable extended FILE facility.
.sp
.LP
The environment of the executed command will be as if a child process were
created within the \fBpopen()\fR call using \fBfork\fR(2).  The child is
created as if invoked with the call:
.sp
.LP
\fBexecl("/usr/bin/sh", "sh", "\fR\fB-c\fR\fB", \fR\fIcommand\fR, \fB(char
*)0);\fR
.sp
.LP
The \fBpclose()\fR function closes a stream opened by \fBpopen()\fR by closing
the pipe. It waits for the associated process to terminate and returns the
termination status of the process running the command language interpreter.
This is the value returned by \fBwaitpid\fR(3C). See \fBwait.h\fR(3HEAD) for
more information on termination status. If, however, a call to \fBwaitpid()\fR
with a \fIpid\fR argument equal to the process ID of the command line
interpreter causes the termination status to be unavailable to \fBpclose()\fR,
then \fBpclose()\fR returns \(mi1 with \fBerrno\fR set to \fBECHILD\fR to
report this condition.
.SH RETURN VALUES
.sp
.LP
Upon successful completion, \fBpopen()\fR returns a pointer to an open stream
that can be used to read or write to the pipe. Otherwise, it returns a null
pointer and may set \fBerrno\fR to indicate the error.
.sp
.LP
Upon successful completion, \fBpclose()\fR returns the termination status of
the command language interpreter as returned by \fBwaitpid()\fR.  Otherwise, it
returns \fB\(mi1\fR and sets \fBerrno\fR to indicate the error.
.SH ERRORS
.sp
.LP
The \fBpclose()\fR function will fail if:
.sp
.ne 2
.na
\fB\fBECHILD\fR\fR
.ad
.RS 10n
The status of the child process could not be obtained, as described in the
DESCRIPTION.
.RE

.sp
.LP
The \fBpopen()\fR function may fail if:
.sp
.ne 2
.na
\fB\fBEMFILE\fR\fR
.ad
.RS 10n
There are currently \fBFOPEN_MAX\fR or \fBSTREAM_MAX\fR streams open in the
calling process.
.RE

.sp
.ne 2
.na
\fB\fBEINVAL\fR\fR
.ad
.RS 10n
The \fImode\fR argument is invalid.
.RE

.sp
.LP
The \fBpopen()\fR function may also set \fBerrno\fR values as described by
\fBfork\fR(2) or \fBpipe\fR(2).
.SH USAGE
.sp
.LP
If the original and \fBpopen()\fR processes concurrently read or write a common
file, neither should use buffered I/O. Problems with an output filter may be
forestalled by careful buffer flushing, for example, with \fBfflush()\fR (see
\fBfclose\fR(3C)). A security hole exists through the \fBIFS\fR and \fBPATH\fR
environment variables.  Full pathnames should be used (or \fBPATH\fR reset) and
\fBIFS\fR should be set to space and tab (\fB" \et"\fR).
.sp
.LP
Even if the process has established a signal handler for \fBSIGCHLD\fR, it will
be called when the command terminates.  Even if another thread in the same
process issues a \fBwait\fR(3C) call, it will interfere with the return value
of \fBpclose()\fR. Even if the process's signal handler for \fBSIGCHLD\fR has
been set to ignore the signal, there will be no effect on \fBpclose()\fR.
.SH EXAMPLES
.LP
\fBExample 1 \fR\fBpopen()\fR example
.sp
.LP
The following program will print on the standard output (see \fBstdio\fR(3C))
the names of files in the current directory with a \fB\&.c\fR suffix.

.sp
.in +2
.nf
#include <stdio.h>
#include <stdlib.h>
main(\|)
{
        char *cmd = "/usr/bin/ls *.c";
        char buf[BUFSIZ];
        FILE *ptr;

        if ((ptr = popen(cmd, "r")) != NULL) {
                while (fgets(buf, BUFSIZ, ptr) != NULL)
                        (void) printf("%s", buf);
                (void) pclose(ptr);
        }
        return 0;
}
.fi
.in -2

.LP
\fBExample 2 \fR\fBsystem()\fR replacement
.sp
.LP
The following function can be used in a multithreaded process in place of the
most common usage of the Unsafe \fBsystem\fR(3C) function:

.sp
.in +2
.nf
int my_system(const char *cmd)
{
        FILE *p;

        if ((p = popen(cmd, "w")) == NULL)
                return (-1);
        return (pclose(p));
}
.fi
.in -2

.SH ATTRIBUTES
.sp
.LP
See \fBattributes\fR(7) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	See below.
_
MT-Level	Safe
.TE

.sp
.LP
The \fBF\fR character in the \fImode\fR argument of \fBpopen()\fR is Evolving.
In all other respects this function is Standard. The \fBpclose()\fR function is
Standard.
.SH SEE ALSO
.sp
.LP
.BR ksh (1),
.BR pipe (2),
.BR fclose (3C),
.BR fopen (3C),
.BR posix_spawn (3C),
.BR stdio (3C),
.BR system (3C),
.BR wait (3C),
.BR waitpid (3C),
.BR wait.h (3HEAD),
.BR attributes (7),
.BR standards (7)
